FIG. 1 shows a cross-section of an ordinary optical disc, designated as 10. A recording layer 2 is formed on a transparent substrate 1 of resin, such as, for example, polycarbonate, and a protective film 3 is formed over the recording layer 2. The substrate 1, the layer 2 and the film 3 form a disc unit 4. Two such disc units 4 are bonded to each other at the exposed surfaces of the respective protective films 3 with an adhesive layer 5. Thus, the optical disc 10 is formed. Conventionally, in order to bond the disc units 4 together, a thermosetting adhesive, an ultraviolet-curable adhesive, or a thermoplastic adhesive are used. A corrosive component contained in such adhesives, however, tends to degrade the recording layers 2, and, in particular, when the water absorption of the adhesive layer 5 is high, the protective films 3 and the recording layers 2 are oxidized, which lowers the reliability of the disc 10. A moisture-curing adhesive, such as one-pack epoxy resin adhesive, may be used. However, when disc units having large areas are bonded with a thin layer of a moisture-curing adhesive, air or moisture hardly penetrates to reach center portions of the bonded disc units, resulting in incomplete curing of the adhesive. Furthermore, bonding disc units with such an adhesive requires a long time and is difficult to do. When two-pack non-mixing type resins and microcapsule type resins are used, some components may remain uncured, which will erode the discs. Furthermore, such adhesives can provide insufficient adhesion. For such reasons, two-pack non-mixing type and microcapsule type adhesives are considered unsuitable for bonding disc units.
Another problem which may be encountered when curing type adhesives are used is that contraction of the curing adhesive may cause distortion of the disc units. In particular, when a thermosetting adhesive is used, not only does contraction of the adhesive occur when it cures, but also heat applied for curing the adhesive distorts the adhesive layer 5 itself. Distortion of the adhesive layer 5 causes deformation or warpage of the disc units, and, therefore, resulting optical discs may have to be rejected.
The use of a thermoplastic resin adhesive rather than a thermosetting resin as the adhesive layer 5 is disclosed in, for example, Japanese Patent Publication No. SHO 63-67258. In the invention disclosed in this patent publication, a hot-melt type adhesive, which is thermoplastic, is used for the adhesive layer 5. A hot-melt adhesive which has been heated and melted is applied over the protective film 3 of a first disc unit 4. Then, the other disc unit 4 is placed on the first disc unit 4 in such a manner that the protective film 3 of the other disc unit 4 comes into contact with the adhesive which has been applied over the protective film 3 of the said first disc unit 4. Then, the assembly is cooled to room temperature so that the hot-melt adhesive cures to bond the two disc units 4 to each other.
Since such a thermoplastic resin adhesive need not be heated for its curing, distortion of the adhesive layer 5 is small, which, in turn, can advantageously reduce warpage of the disc units 4. However, if the adhesive is applied non-uniformly, portions of the adhesive layer 5 may swell from to moisture which penetrates through the substrate 1, and the disc units may crack or may separate from each other.
Usually, optical discs are used in different environments, from cold districts to hot districts, and, accordingly, the temperature at which discs are designed to perform desired functions (hereinafter this temperature is referred to as usable environment temperature) ranges, for example, from -20.degree. C., to 60.degree. C. The inventors have conducted humidity-resistance tests and temperature-humidity cycle tests on optical discs comprising two disc units 4 bonded together with a hot-melt adhesive at the above-stated temperature range of from -20.degree. C. to 60.degree. C. at which discs may be used. They found that the recording layers 2 were readily peeled off, pin holes were produced and, when the number of test cycles increased, bit error rates increased abruptly. Therefore they concluded that such optical discs were not sufficiently reliable.
One conventional technique for bonding two disc units 4 together is as follows. A first disc unit 4 is positioned with the recording layer 2 facing upward. An adhesive is applied over the protective film 3 on the recording layer 2 in generally concentric circles. A second disc unit 4 is aligned with the first disc by means of a center shaft of a disc manufacturing apparatus, and the protective film 3 of the second disc unit is brought into contact with the adhesive. The resultant assembly is left as it is so that the the adhesive is spread over the entire surfaces of the protective films due to the weight of the disc unit, and the adhesive is caused to cure to bond the two disc units together. In this bonding technique, bubbles may be disadvantageously formed in the adhesive when it is applied over the protective film or when the second disc unit is brought into contact with the adhesive on the protective film of the first disc unit, and the bubbles may remain after the adhesive cures.
One method for preventing such bubbles from being formed in the adhesive layer is shown in Japanese Unexamined Patent Publication No. SHO 61-50231. According to the method shown in this patent publication, an adhesive is applied over the entire surface of the protective film 3 by spin-coating, and the two disc units are bonded together with their center axes aligned with each other. According to another method which is shown in Japanese Unexamined Patent Publication No. SHO 61-292244, an adhesive is applied over portions of the protective film 3 of one disc unit, the other disc unit is placed over the first disc unit with the center axes of the two disc units aligned, and pressure is applied to bond the two disc units together with the adhesive spread over the entire surfaces of the protective films.
When an adhesive is applied over the entire surface of the protective film 3 of one disc unit 4 and the other disc unit is bonded to the first disc unit, the adhesive may forced out into the center holes of the disc units and also around the bonded disc units, and, when the adhesive cures, burrs 7 may be formed on the periphery of the center hole 6 of the optical disc 10 and on the periphery of the disc 10, as shown in FIGS. 2(a) and 2(b). Burrs 7 formed within the center hole 6 could make the disc 10 eccentric, and, accordingly, they should be completely removed. In order to deburr, a deburring device, such as one shown in Japanese Unexamined Patent Publication No. SHO 61-80534, may be used. However, the use of a deburring device will undesirably increase the number of manufacturing steps, which, in turn, increases the manufacturing costs. On the other hand, even when the adhesive is applied over portions of the protective film, it may be forced out or ooze out as in the case when the adhesive is applied over the entire surface as stated above, or, sometimes, the distribution of the adhesive may be non-uniform, so that the disc 10 may flutter.
In order to prevent the adhesive from oozing out into the center hole or out of the periphery of the disc, a precisely adjusted pressure must be applied, which requires high-precision, expensive equipment. One of the simplest techniques for bonding two disc units is the use of the weight of a disc unit itself with an adhesive placed between the two disc units. In this technique, however, if an adhesive having viscosity of less than 100 cps is used, it may ooze out and form burrs 7 like the ones shown in FIGS. 2(a) and 2(b). In contrast, if the viscosity of the adhesive is above 1000 cps, the adhesive may not spread over the entire space between the two disc units 4, as shown in FIGS. 3(a) and 3(b). Even if the amount of the adhesive to be applied is precisely measured, oozing out of the adhesive as shown in FIGS. 2(a) and 2(b) or absence of the adhesive at some portions as shown in FIGS. 3(a) and 3(b) may occur if the adhesive is applied to disc units at inappropriate positions.
A reduced-pressure bonding apparatus as shown in FIG. 4 has been conventionally used for bonding, with an adhesive, two disc units without leaving bubbles in the adhesive layer. In FIG. 4, a vacuum chamber 11 houses mounts 14 and 15 coupled respectively to shafts 12 and 13 which can move up and down. Disc units 4 with adhesive layers 16 and 17 applied over the surfaces of protective films 3 of the respective disc units 4 are mounted on the mounts 14 and 15, respectively. A vacuum pump (not shown) is operated to reduce the pressure in the vacuum chamber 11 through an exhaust pipe 18 to a pressure of about 20 Torr or less. Then, the shafts 12 and 13 move the mounts 12 and 13 toward each other for bonding the disc units 4 together. The adhesive may be cured under a reduced pressure or under normal pressure, but the pressure under which the bonding of disc units is carried out must be about 20 Torr or below. When the pressure is higher than that, bubbles may in the adhesive.
When the reduced-pressure bonding apparatus of FIG. 4 is used, the adhesive is applied to at least portions of the protective films 3 of the disc units 4, and the disc units 4 are bonded with the adhesive which is spread over the entire surfaces of the films 3 due to application of pressure. It is, therefore, necessary to control precisely the movement of the shafts 12 and 13 in order to prevent the adhesive from oozing out into the center hole or to the outer periphery of the disc or from being non-uniformly distributed. Furthermore, it is necessary to determine precisely the amount of adhesive to be applied and also the position where the adhesive is to be applied. In addition, it is also necessary to maintain the pressure in the reduced-pressure bonding apparatus at about 20 Torr or below. If one wants to use this type of apparatus for mass-production of optical discs, the size of the apparatus must be large.
Therefore, a first object of the present invention is to produce optical discs which are free of warpage of the discs and peeling off of a recording layer of the discs when they are used at a usable environment temperature within a range of, for example, from -20.degree. C. to 60.degree. C.
A second object of the present invention is to produce, at low costs, highly reliable optical discs in which degradation of recording layers and deformation of disc units are eliminated by the use of an adhesive that can reduce the number of discs rejected in the step of bonding two disc units.
A third object of this invention is to provide an improved method of manufacturing optical discs, according to which, when two disc units are bonded together with an adhesive, the adhesive does not ooze out into the center hole or to the outer periphery of a resulting disc, and, accordingly, a step for removing burrs can be eliminated. The resulting optical discs are free of eccentricity and free of surface fluttering.